1. Field of the Invention
The present invention relates to a hydraulic control system of an automatic transmission, in particular, to a control system of an engaging force of frictional elements during a shift operation.
2. Related Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift speed mechanism which is combined with the torque converter. A plurality of frictional elements of the shift speed mechanism is selectively switched to change a power transmitting path to establish a predetermined shift stage in accordance with a driver's need and/or an driving condition. For this purpose, the automatic transmission is provided with a hydraulic control circuit which produces a line pressure for engaging the frictional elements. If the line pressure is too low compared with an input torque for a frictional element, a torque transmission amount of the frictional element is short to transmit a desired torque amount. Conversely, when the line pressure is too high, for instance, a torque amount for driving an oil pump is increased beyond an appropriate level to consume an engine output undesirably. This means the fuel consumption efficiency is deteriorated.
In view of this, the line pressure is controlled based on a throttle valve opening by which the engine output torque is controlled because the input torque introduced to the frictional element generally corresponds to the engine output torque. As a result, the line pressure is controlled so as to match the line pressure to the input torque of the frictional element.
Meanwhile, in order to avoid a substantial change in a vehicle speed during the shift operation, an input speed (turbine speed ) is changed so as to a target gear ratio. In an up-shift operation in which the input speed is reduced, a torque amount of the frictional element involved in the shift operation is increased by an amount for an inertia torque which is necessary to reduce the turbine speed. On the contrary, in a down-shift condition in the case where the input speed is increased, the torque amount of the frictional element involved in the shift operation is reduced by an amount for the inertial torque which is necessary to increase the input speed.
Thus, an optimized line pressure cannot be obtained only by setting the line pressure based on the input torque introduced to the shift speed mechanism. As a result, the line pressure does not correspond to the required hydraulic pressure for the frictional element. For example, when the line pressure is too high relative to the required pressure, the shift operation period is undesirably shortened to produce an uncomfortable shift shock. When the line pressure is too low relative to the required hydraulic pressure, the shift operation period is undesirably elongated so that a shift feeling is deteriorated.
In order to deal with the above problem, Japanese Patent post-examination publication No. 4-74099 discloses that a operation pressure or the line pressure is determined taking account of the input torque introduced to the shift speed mechanism and the inertia torque caused by a speed change in the shift speed mechanism during the shift operation. This technique focuses on the fact that the engine speed is changed so as to correspond to the output speed of the shift speed mechanism. According to the above control disclosed in the Japanese publication No. 4-74099 which was opened to the public in 1992, it is expected that the shift operation can be completed in a shortened time period without producing an undesirable shift shock.
On the other hand, Japanese Post-examination publication No. 1-98745, which was opened to the public in 1989, discloses a control in which a change rate of the input speed of the shift speed mechanism is detected so that the working hydraulic pressure (including the line pressure) for a next shift operation is compensated by means of a learning control based on a difference between the change rate and a predetermined target speed change rate. Japanese Post-examination publication No. 1-15006, which was opened to the public in 1989, discloses a control in which the operating hydraulic pressure in a later stage of the shift operation for a hydraulic circuit having an accumulator damping the is operating pressure introduced to the frictional elements compensated by means of the learning control based on the comparison between the speed change rate in the shift speed mechanism and a predetermined speed change rate.
There is a following problem in the control as disclosed in the Japanese patent publication No. 4-72099 in which the operating pressure is determined based on the input torque introduced to the shift speed mechanism and the inertia torque.
The characteristics of the automatic transmissions depend on the products due to variations between elements thereof as well as the engines which are combined with the automatic transmissions. Thus, in order to obtain the optimized shift speed characteristics, it is necessary to tune each of the combinations of the automatic transmission and engine. Otherwise, the working efficiency is deteriorated. Even if the optimized tuning is made at the beginning, the shift speed characteristics are changed as the time goes.
In order to cope with this problem, it is proposed that the operating pressure applied to the frictional element during the shift operation is compensated by means of the learning control utilizing the control as disclosed in the Japanese patent publications Nos. 1-98745 and 1-150056.
It should, however, be noted that the compensation is made for the hydraulic pressure corresponding to the total value of the input torque and inertia torque in the control as disclosed in the above Japanese patent publication. Consequently, for example, the result of the learning control in the high altitude where an intake air density is relatively low is different from that in the low altitude where the intake air density is relatively high. As a result, the shift operation time period is slightly changed, for example, just after the vehicle is moved between the low and high altitudes. This means that such control cannot follow up the change of the circumstance.
In addition, the conventional learning control produces a relatively large variation in the result of the learning control so that a sophisticated result of the learning control cannot be obtained.